Foam backing will be needed to press the PCB evenly against a photomask. Otherwise, acrylic (and similar) frames may bubble under pressure, allowing the photomask to 'pop-up' in the middle.

During chemical processing, alternate between photodeveloper and etchant. Proceed with etchant only when all exposed regions are changing color. Otherwise, problems with ultrathin photoresist residue may remain.

Avoid placing UV light sources in extreme proximity to the board. Doing so may unevenly expose some regions.

Photosensitive copper clad board. If not already available, more can be made by laminating dry film photoresist to bare copper clad board.

Via/hole drilling, outline milling, etc. Best done automatically on CNC mill. Please be careful to terminate outline cutting once complete, do not allow the endmill to bind.

Photomask. Printed on transparencies at high-resolution. Inkjet printers, >600dpi, do well, given true inkjet transparencies. Some laser printers may require double layer photomasks. Alternatively, <445nm lasers may be able to selectively expose the photoresist.

UV exposure. Sunlight or high-brightness CFL bulbs can expose the photoresist in ~5-30 minutes. EEPROM eraser boxes with timers are best. Even exposure is critical, and timing is critical for lower-contrast photomasks.

Development. Use a foam brush with the appropriate developer solution from the chemicals shelf. Alternatively, a warm bath can quickly provide better consistency across large boards, particularly if photodeveloper includes silicate.

Etching. Recommend a sponge with ferric chloride to remove exposed copper. This is quicker and less wasteful than the "bath" process.

For large boards with hundreds of components, reflow can be quicker than manual soldering.

Laser cut solder paste stencil from blue tape. Apply stencil to PCB.

Add smooth, paper thin layer of solder paste with a razor.

Remove stencil.

Place components on the appropriate pads.

Carefully melt the solder with the hot-air reflow tool. Hold the hot-air tool close to the components at 400C and maximum airflow, watch carefully, and progressively move the tool across the board as solder melts.

Note that this process is somewhat incompatible with hand-soldered vias.

Lots of copper dissolving etchant, various types, some good, some bad.

CNC Mill.

A heated and bubbled etchant bath has been constructed by mirage335. While reasonably quick (~15 minutes) and consistent, this is not recommended, as components degrade quickly, and the resulting acid vapor is extremely harsh.

Please be careful with the chemicals, most of which are strong acids or bases. Strictly avoid contact and vapors. Nitrile gloves are available. Immediately rinse and move to fresh air after any accidental contact.

Generally, the most hazardous operations involve the etchant bath, and preparing fresh photodeveloper. The actual PCB fabrication process itself, applying an etchant bottle to a sponge and using non-concentrate photodeveloper, is relatively simple.

Better photodeveloper. The MG Chemicals product works reasonably well thus far, but may not include sodium silicate. [Reportedly] RS-Components 690-849 is far superior, allowing development in seconds with lower risks of overdevelopment.

Professional PCB fabs manufacture upwards of dozens of PCBs per hour, with resulting boards ready for rapid solderpaste stencil based PCB assembly. Efforts are underway to implement required DLP projection and spray tank equipment for at least part of this process, a description of which follows:

Similar to dual-layer boards. However, interior layers are processed first as two sided PCBs, with exterior processing and through-hole plating taking place after inter-layer bonding. Preferably, drilling also takes place after inter-layer bonding.

Predrill registration holes in panel corners. In additional to presensitized board, pre-preg should also be predrilled.

Predrill all holes if dual pre-aligned DLP projection is not available. In additional to presensitized board, pre-preg should also be predrilled. Beware separately predrilled layers may fail to electrically connect in multilayer boards.

Exterior layers should now be fully processed, down to thick patterned copper. Through-holes should have been plated along with exterior layers. Interior layers are completed, with via copper edges adjoining the through-hole plating. Tin plating should have protected the through-holes and associated copper joints.

At this point, the multilayer board should be physically and electrically flawless, ready for high-temperature soldering.

Minimum cost to implement the full system is therefore around $700, a $1k budget is probably reasonable. Better projectors may be worth investing in, to ensure highly successful fabrication results are achieved.

Cupric Chloride (typically the green stuff) should be used as the etchant, as it can be recharged completely with atmospheric oxygen and hydrochloric acid. However, Ferric Chloride should be used initially, as it performs better with less tendency to 'grow'.